Semiconductor chip package assembly with deflection-resistant leadfingers

ABSTRACT

The invention relates to leadframes and semiconductor chip package assemblies using leadframes, and to methods for their assembly. A disclosed embodiment of the invention includes a semiconductor package leadframe with a chip mounting surface for receiving a semiconductor chip and a plurality of leadfingers. The leadfingers have a proximal end for receiving one or more wirebond, and a distal end for providing an electrical path from the proximal end. One or more of the leadfingers also has an offset portion at its proximal end for increasing the clearance between the leadfinger and underlying heat spreader, increasing the stiffness of the leadfinger, and increasing leadfinger deflection-resistance and spring-back. The offset is in the direction opposite the plane of a heat spreader thermally coupled to the mounting surface. Preferred embodiments of the invention further include a semiconductor chip affixed to the mounting surface and a plurality of bondwires operably coupling bond pads of the chip to the offset portions of the proximal ends of individual leadfingers.

This is a division of application Ser. No. 12/120,515 filed May 14,2008, U.S. Pat. No. 7,928,544, the contents of which are hereinincorporated by reference in its entirety.

TECHNICAL FIELD

The invention relates to electronic semiconductor chip packageassemblies and manufacturing. More particularly, the invention relatesto package assemblies and associated methods for manufacturingsemiconductor chip package assemblies with improved leadframes andimproved processes for wirebonding leads to leadframes.

BACKGROUND OF THE INVENTION

In leaded semiconductor chip packages, a chip is generally affixed to apaddle on a leadframe using a permanent adhesive. In some cases, it isdesirable to enhance the thermal performance of a package by improvingthe path for the departure of heat from the chip through the leadframe.In many such cases, it is known to attach a heat spreader to the surfaceof the paddle opposite the chip, such as by welding, taping, or gluing.In other cases, the leadframe paddle is dispensed with, using instead aheat spreader riveted in place on the leadframe. The chip is thenmounted directly to the heat spreader surface. In either case, due toits heat conduction properties, the heat spreader is typically made frommetal, such as copper or copper alloy. Generally, in order to increaseits heat conduction, the heat spreader is large relative to the chip,extending in a plane parallel with the proximal ends of the leadfingers.The leadframe is typically planar, with leadfingers extending in astraight path from the edges toward the chip location. This type ofarrangement of chip, leadfingers, and heat spreader at least partiallyunderlying the leadfingers, is used in many applications, but is notwithout its problems.

Electrical connections within a semiconductor chip package are commonlymade by bonding wires from bond pads on the exposed surface of the chipto the leadfingers. The leadfingers typically extend in a straight linefrom one end adjacent to a gap in the leadframe proximal to the chip, toa distal end at the exterior of the package where electrical connectionsmay be made to the outside world. During a typical wirebonding process,a ball bond is formed on a bond pad of the chip using heat, pressure,and in many cases ultrasonic vibrations. The wire is then pulled to theappropriate proximal end of a leadfinger, and a stitch bond is formedthere, also using some combination of heat and pressure, and oftenultrasonic vibrations.

In package assemblies having a heat spreader extending parallel to theproximal ends of the leadfingers, it is often impractical to support theleadfingers during wirebonding. In some applications, where support toleadfingers during wirebonding is deficient, it is known to provideincreased clearance between the leadfingers and the underlying heatspreader by increasing the distance between them, or by downsetting thechip pad away from the plane of the leadfingers. The result in suchcases is thicker package assemblies, which in most applications isundesirable. On the other hand, minimizing thickness by reducing suchclearance can result in electrical problems and defective packageassemblies.

Due to these and other technological problems, improved leadfingers,leadframes, semiconductor chip package assemblies, and methods for theirmanufacture would provide useful and advantageous contributions to theart. The present invention is directed to overcoming, or at leastreducing, problems present in the prior art, and contributes one or moreheretofore unforeseen advantages indicated herein.

SUMMARY OF THE INVENTION

In carrying out the principles of the present invention, in accordancewith preferred embodiments thereof, the invention provides novel anduseful improvements for leadframes used in semiconductor chip packageassemblies, and related methods. Experience, observation, analysis, andcareful study of defects in semiconductor device packages related toclearance issues have led the Applicants to determine that particularproblems are encountered in applications having leadfingers extending ina plane parallel to a heat spreader. Typically, the proximal end of theleadfinger, which is suspended parallel to the heat spreader and lackslateral support, is deflected “downward”, e.g., in the directionopposite the wirebond, by the application of pressure from thewirebonding tool. In some cases, the deflected leadfinger comes intocontact with the underlying heat spreader during wirebonding. Due to themechanical properties of the leadframe material, however, which istypically made from metal such as aluminum, copper, or alloy, theleadfinger has some capacity to spring back toward its original shapeafter the pressure of the bonding tool is removed. The return of theproximal end of the leadfinger toward its original position is often notcomplete, however. In some, more problematic, applications, multiplewirebonds may be formed from a chip to a single leadfinger. It has beenobserved that in such cases, the effects of repeated deflection of theleadfinger may be cumulative, with the result that the proximal end ofthe leadfinger becomes permanently deformed by the wirebonding process.When the geometry of the package includes a heat spreader extending in aplane parallel to the proximal ends of the leadfingers, the proximalends of the leadfingers, particularly those bearing multiple bondwires,can in some cases be left in contact with, or nearly in contact with,the heat spreader due to deformation caused by the pressures appliedduring wirebonding. In such cases, undesirable interference,capacitance, or even short circuits may result. In order to circumventthis problem, the Applicants have contrived to configure the leadfingersoffset from the plane of the leadframe in a novel way in order toincrease clearance between leadfingers and heat spreaders withoutincreasing overall package thickness. Such endeavors have further led tothe development of leadfinger configurations that not only increaseclearance between leadfingers and heat spreaders, but also increase themechanical stiffness and spring force of the leadfingers.Synergistically, the invention provides leadfinger configurations thatincrease clearance within a given package thickness, while providingimproved stiffness and “spring-back”. Mechanically speaking, stiffnessis a property of a solid body dependent on both the properties of thematerial, such as elastic modulus, and the shape of the solid body. Fora solid body in compression, the axial stiffness may be expressed as,the product of the cross-sectional area and the modulus of elasticity(aka, Young's modulus) of the material, divided by the length of thebody. The spring force exerted by the body is defined by the product ofthe stiffness and the distance it may be moved. In the case of theinvention, it is believed that the stiffness and springiness of theleadfingers may be increased by offsetting a portion of the leadfingers.

For the purposes of avoiding confusion and unnecessary repetition infully describing the invention, the term “mounting surface” is usedherein to refer to the surface upon which a chip may be mounted, whichincludes a chip paddle integral with the leadframe, or a portion of aheat spreader adapted to receive the mounting of a chip directly on aportion of its surface.

According to one aspect of the invention, in an example of a preferredembodiment, a semiconductor chip package leadframe has a mountingsurface adapted for receiving a semiconductor chip, and a number ofleadfingers. Each of the leadfingers has a proximal end for receivingone or more wirebond and a distal end for providing an electrical pathfrom the proximal end. At least some of the leadfingers also have anoffset portion at the proximal end, the offset being in the directionopposite the plane of the mounting surface.

According to another aspect of the invention, a semiconductor chippackage assembly includes a metallic leadframe with a mounting surfacefor receiving a semiconductor chip. A heat spreader is thermally coupledwith the mounting surface. A semiconductor chip with numerous bond padson its exposed surface is affixed to the mounting surface. The leadframealso includes a plurality of leadfingers, each having a proximal end forreceiving one or more wirebond, and a distal end for facilitatingelectrical connections external to the package. A number of theleadfingers also have an offset portion at the proximal end, in thedirection opposite the plane of the heat spreader. Bondwires operablycouple bond pads of the semiconductor chip to the offset portions of theproximal ends of individual leadfingers. The assembly, i.e., chip,bondwires, heat spreader, and the offset portions of the leadfingers, issubstantially encapsulated in dielectric material.

According to another aspect of the invention, the semiconductor chippackage assembly as mentioned above, and described elsewhere herein,includes at least one leadfinger offset portion which has a plurality ofbondwires attached.

According to still another aspect of the invention, a method forassembling a semiconductor chip package includes the step of providing ametallic leadframe having a mounting surface for receiving asemiconductor chip. A heat spreader is thermally coupled to the mountingsurface. The leadframe also has numerous leadfingers, each with aproximal end for receiving one or more wirebond, and a distal end forreceiving electrical connections external to the package. One or more ofthe leadfingers includes an offset portion at its proximal end, theoffset being in the direction opposite the plane of the heat spreader.In further steps, a semiconductor chip is affixed to the mountingsurface, and bondwires are operably coupled between bond pads of thesemiconductor chip and offset portions of the proximal ends of theleadfingers. Encapsulating the assembly is a further step in completingthe package.

According to yet another aspect of the invention, the method describedfurther includes operably coupling more than one bondwire to a singleleadfinger offset portion.

The invention has advantages including but not limited to one or more ofthe following: decreased thickness in package structures; increasedyield and reliability in manufacturing processes; improved thermalperformance in packages; and reduced cost. These and other features,advantages, and benefits of the present invention can be understood byone of ordinary skill in the arts upon careful consideration of thedetailed description of representative embodiments of the invention inconnection with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will be more clearly understood from considerationof the following detailed description and drawings in which:

FIG. 1 is a cutaway side view of an example of a preferred embodiment ofa semiconductor package leadframe according to the invention;

FIG. 2 is a cutaway side view of an example of an alternative preferredembodiment of a semiconductor package leadframe according to theinvention;

FIG. 3 is a cutaway side view of an example of a preferred embodiment ofa semiconductor chip package assembly according to the invention;

FIG. 4A is top view of an example of a preferred embodiment of asemiconductor chip package assembly according to the invention;

FIG. 4B is partial top view of a portion of the example of a preferredembodiment of a semiconductor chip package assembly shown in FIG. 4A;and

FIG. 5 is a cutaway partial side view of an example of a preferredembodiment of a semiconductor chip package assembly and method accordingto the invention.

The drawings are not to scale, and some features of embodiments shownand discussed are simplified or amplified for illustrating principlesand features, as well as anticipated and unanticipated advantages of theinvention.

DESCRIPTION OF PREFERRED EMBODIMENTS

While the making and using of various exemplary embodiments of theinvention are discussed herein, it should be appreciated that thepresent invention provides inventive concepts which can be embodied in awide variety of specific contexts. It should be understood that theinvention may be practiced with semiconductor package assemblies andassociated manufacturing processes of various types and materialswithout altering the principles of the invention. For purposes ofclarity, detailed descriptions of functions and systems familiar tothose skilled in the semiconductor chip, packaging, and manufacturingarts are not included.

In general, the invention provides semiconductor chip assemblies, andassociated methods, using leadframes having leadfinger offsets toparticular advantage, especially in applications wherein leadfingerdeflection during wirebonding may be a concern. Features of theinvention are advantageous in terms of reduced defects in completedpackage assemblies, increased mechanical strength and durability,improved thermal performance, decreased assembly thickness, and improvedelectrical performance.

Referring initially to FIG. 1, a cutaway side view of an example of aleadframe assembly 28 of the invention for use in a semiconductor chippackage assembly is illustrated. The leadframe 10 is made from metalsuch as aluminum or copper as known in the art, and has a mountingsurface 12, in this exemplary embodiment, the surface 12 of a heatspreader 26 riveted in place on the leadframe 10, for receiving asemiconductor chip 32. Thus, a thermal path is provided from the chip 32to the bottom surface 27 of the heat spreader 26. The leadframe 10 alsohas a number of leadfingers 14 preferably extending from a gap 16 at theends 18 proximal to the mounting surface 12, adjacent to the eventualchip 32 location, outward to where they may ultimately be used asexternal package leads at their distal ends 20. The leadfingers 14preferably each include an offset portion 22, which includes theproximal end 18, which is connected to the distal end 20 by anoffsetting portion 24, typically angled away from the plane of thedistal end 20. Clearance 30 is thus established between the offsetportions 22 of the leadfingers 14 and the heat spreader 26. It isbelieved that when an offset portion 22 is included on the leadfingers14, the proximal ends 18 of the leadfingers 20 are more resistant todeflection by wirebonding tools during wirebonding, and more resilientlyspring back toward their original positions after such deflecting forcesare abated. It is believed that the offset portion 22 of the leadfinger14 increases the not only the clearance 30 between the leadfinger 14 andthe heat spreader 26, but also increases the mechanical stiffness of theleadfinger 14, and that the combined stiffness and clearance 30 furtherincrease the spring force of the leadfinger 14 in resistance tocompression during wirebonding.

The offset portion 22 of a leadfinger 14 according to the practice ofthe invention is offset in the direction most beneficial to electricaland thermal performance. Now referring primarily to FIG. 2, a cutawayside view of a semiconductor package leadframe 10 according to theinvention is shown in a preferred embodiment in which the mountingsurface 12 is a paddle portion integral with the rest of the leadframe14. A heat spreader 26 is attached to the chip paddle mounting surface12 forming a direct thermal path from the mounting surface 12. Theleadframe assembly 28 thus includes leadfingers 14 each having an offsetportion 22 at their proximal ends 18, adjacent to the gap 16 separatingthe leadfinger 14 from the ultimate chip 32 location on the mountingsurface 12 to the opposite surface 27 of the heat spreader 26. Theoffset portion 22 is offset by an angle or bend 24 in the directionopposite the plane of the surface of the heat spreader 26. The amount ofoffset may be adjusted for individual applications based upon factorssuch as the cross-sectional area of the leadfinger, the thickness of thebondwire to be used, the number of bondwires to be attached, and theinherent stiffness of the leadframe material. Preferably, the gap 16between the proximal end 18 of the leadfinger 14 and the chip 32location on the mounting surface 12 is as small as electricallypractical, as it is generally advantageous to minimize the length of thebondwires ultimately used in the package. The enhanced resilience of theleadfinger 14 due to the offset portion 22, and the clearance 30provided between the heat spreader 26 and the offset portion 22 of theleadframe 14, and in some cases the reduced gap 16, provide practicaladvantages described herein.

Depicted in FIG. 3, a cutaway side view of an example of a preferredembodiment of a semiconductor chip package assembly 34 illustratesaspects of the invention. The metallic leadframe assembly 28 preferablyincludes a mounting surface 12 capable of receiving a semiconductor chip32, in this case the central region of a the surface of a heat spreader26 is used. The semiconductor chip 32 preferably has numerous bond pads36 on its exposed surface for connecting bondwires 38 to the offsetportions 22 of the proximal ends 18 of the leadfingers 14. In order thatthe bondwires 38 do not impede the leadfingers 14 from springing back,the bondwires 38 are preferably thin relative to the leadfingers 14,e.g., a 1 mil diameter wire is preferred with a 5 mil thick leadfinger,for example. Providing an additional advantage of the invention, the gap16 between the distal end 18 of the leadfinger 14 and the chip 32 may insome cases be reduced, so that the length of the bondwires 38 may inturn be minimized in order to improve electrical performance. The heatspreader 26, as shown in the drawing, provides a thermal path from the“bottom” (as oriented in the drawings) of the chip 32 to the heatspreader surface 27 at the exterior of the package assembly 34. It canbe seen in this example of a package assembly 34 according to apreferred embodiment of the invention that the offset 22 of the proximalends 18 of the leadfingers 14 provides clearance 30 between the heatspreader 26 and the leadfingers 14. The offsetting portion 24 of theleadfingers 14 facilitates not only creating clearance 30 between theproximal ends 18 of the leadfingers 14 and the heat spreader 26, butalso increases the leadfingers' 14 mechanical resistance to deflectionby the application of force by wirebonding tools used for forming a bond40 on the offset portion 22, and increases stiffness and resilience, or“spring-back”, of the offset portion 22 subsequent to deflection. It isbelieved that the offset 22 increases the mechanical stiffness of theleadfinger 14 by effectively shortening it's length relative to thedownward force applied by the wirebonding tool. It is also believed thatthe increased clearance 30 provided by the offset portion 22, incombination with the increased stiffness of the leadfinger 14, increasethe spring-back capability of the leadfinger 14; Spring force is equalto the product of the stiffness and the clearance (30) distance.

It should be appreciated that the invention enables the use of a thickerheat spreader 26 for a given package assembly 34 thickness, and/or athinner package 34 for a given heat spreader 26 thickness. Additionally,the enhancements of the invention enable the use of longer leadfingers14, which makes possible a reduction of the gap 16 between the chip 32and the proximal ends 18 of the leadfingers 14. This, in someapplications, may in turn enable the use of shorter bondwires 38,improving electrical performance. The package assembly 34 is preferablyencapsulated with curable dielectric mold compound 42 such as plastic orepoxy resin as known in the arts. Although variations are possible, theencapsulant 42 typically engulfs the chip 32, bondwires 38, most of theleadframe 10, and the sides of the heat spreader 26, preferably leavingexposed only the distal ends 20 of the leadfingers 14, and “bottom” (inFIG. 3) surface 27 of the heat spreader 26. The invention is believed toexhibit additional unexpected advantages in terms of providing packageassemblies with improved rigidity, strength, and durability due to themold-locking characteristics of the leadfinger offsets.

As shown in the top view of FIGS. 4A and 4B, the invention isadvantageous in that bondwires, e.g. 38, may be wirebonded from the bondpads 36 on the chip 32 to the to offset portions 22 at the proximal ends18 of the leadfingers 14. Multiple bondwires 38, for example, sets oftwo, three, and four bondwires 38 are shown in FIGS. 4A and 4B, may bebonded to a single leadfinger proximal end 18, without exceeding thecapability of the leadfinger 14 to spring back into an acceptableposition relative to the surface of the heat spreader 26. Theleadfingers 14 are preferably configured to optimize the gap 16 betweentheir proximal ends 18 and the chip 32, preferably reducing the requiredlengths of the bondwires 38.

FIG. 5 is an alternative cutaway side view representing an example of apreferred embodiment of a semiconductor chip package 34 and conceptualview of steps in a preferred package assembly method according to theinvention. In practicing the methods of the invention, a metallicleadframe 10 is provided with a chip mounting surface 12, such as inthis example, a portion of the surface of a heat spreader 24, forreceiving a semiconductor chip 32 affixed thereto. The leadframe 10 alsohas a number of leadfingers 14, each with a proximal end 18 forreceiving one or more bondwires 38, and a distal end 20 for makingelectrical connections external to the package 34. At least some of theleadfingers 14 are endowed with an offset portion 22 at the proximal end18, the offset being in the direction opposite the plane of the heatspreader 26 in order to provide clearance 30 between the leadfinger 14and heat spreader 26 opposite. The assembly 34 is encased with curableencapsulant 42 in a manner known in the arts for encapsulating the chip32, bondwires 38, portions of the heat spreader 26, and portions 22 ofthe leadfingers 14. As indicated at arrow “A”, the proximal end 18 ofthe leadfinger 14 may be deflected toward the heat spreader 26, in somecases even making contact, during the formation of a wirebond 40. Theoffsetting portion 24 of the leadfinger 14 provides sufficient stiffnesswhereby the offset 22 at the proximal end 18 of the leadfinger 14 iscapable of springing back to, or nearly to, its original positionsubsequent to one or more wirebonding events. The clearance 30 requiredby the application between the leadfinger 14 and the heat spreader 26can thus be maintained prior to the encapsulation 42 of the package 34.

The methods and package assemblies of the invention provide internalleadfinger offsets on the proximal ends of leadfingers, endowingleadframes and package assemblies with one or more useful advantagesincluding but not limited to surprisingly improved mold lockingproperties, increased rigidity, reduced thickness, improved thermalperformance, increased durability, and reduced costs. While theinvention has been described with reference to certain illustrativeembodiments and particular advantages, those described herein are notintended to be construed in a limiting sense. For example, variations orcombinations of steps or materials in the embodiments shown anddescribed may be used in particular cases without departure from theinvention. Various modifications and combinations of the illustrativeembodiments as well as other advantages and embodiments of the inventionwill be apparent to persons skilled in the arts upon reference to thedrawings, description, and claims.

1. A semiconductor chip package leadframe comprising: a mounting surfacefor receiving a semiconductor chip; a heat spreader for providing athermal path from the mounting surface; a plurality of leadfingers, eachleadfinger having a proximal end for receiving one or more wirebond, theproximal ends of the leadfingers defining a plane parallel to a surfaceof the heat spreader, each leadfinger also having a distal end forproviding an electrical path external to the semiconductor chip packagefrom the proximal end which will be internal to the semiconductor chippackage; wherein, the plurality of the leadfingers each furthercomprises an offset portion at its proximal end, the offset portionbeing in the direction opposite the plane of the heat spreader andangled away from a plane of the distal end.
 2. The leadframe accordingto claim 1 wherein the mounting surface comprises a portion of thesurface of the heat spreader.
 3. The leadframe according to claim 1wherein the mounting surface further comprises a paddle portion of theleadframe.
 4. The leadframe according to claim 1 further comprisingrivets securing the heat spreader to the leadframe.